Several weeks ago, I visited upon a most interesting device.
It uses propane gas to cause a detonation sounding very similar
to a shotgun. The device itself consists of a long tube which is
connected to a propane tank through a very interesting mechanical
timing device. Apparently, farmers use them to ward off wildlife
which is all too eager to consume the fruits of their labor. Being
that in most places these days, any solid fuel noisemaker larger than a
firecracker is illegal without special permit, I became determined to
begin construction of my own. Think of it as a much safer,
reusable alternative to an M80.

Before beginning the project, an
examination of prior art must be made.

Though interesting, the timing device isn't really essential for our
application. The detonations will be set off manually by a remote
so that the operator isn't right next to the unit when detonation
occurs.

Perhaps the most intriguing aspect of this device is that it uses
the same propane that gently heats our homes, dries our clothes, and
cooks our food. Who would've guessed that this same fuel could
produce powerful detonations. As it turns out, propane is
actually one of the more difficult gases to detonate. Other
gases, such as hydrogen, detonate rather easily. The study of
this phenomenon will be central to the project.

Another consideration is that
the commercial version is made from
steel. Since steel is by nature a hard substance, it would be
difficult to work with in the absence of expensive equipment like
spinning lathes, turning lathes, bending tables, cutting torches, etc.
Hence, our material of choice will
be PVC. As can be seen in the above pictures, the steel is
thin anyway, so schedule 80 PVC will be more than adequate.

Update: 7/15/2005

The main firing chamber has
been constructed and there has been good progress on the fuel delivery
system. Unfortunately, the video camera arrived later than I
expected so a text description of the undocumented developments follows:

Initial runs only produced
flames in the barrel. Sometimes the flame would go up the
barrel and produce what can only be described as a humming sound.
Other times, the flame would only sit in the barrel and burn
slowly. Once a flame was produced, it was difficult to ignite
in subsequent runs.

After many hours of head
scratching and some modifications to the fuel delivery system, what I
call a 'thump' was produced on a more consistent basis. The
flame was somewhat easier to ignite, although there was still quite a
bit of difficulty. Although not the intended purpose of the
experiment, various objects were loaded into the barrel and shot out.
The maximum distance traveled was found to be around 50ft,
largely due to the 7/8" hole in the breech where the fuel is admitted.
This is well under the distance which the objects can be
thrown by hand. Most of the hot gases escape out the hole
with a loud 'ssssshhh'. Also, proper threads for the sparking
mechanism were not formed, due to the absence of a proper sized tap.
Thus, in the event that the breech hole is sealed, further
launching distances are expected.

With the idea that many of the problems with difficult ignition were due to the
weak sparking device, which up to this point had been a piezoelectric grill
igniter, the setting out for building of a more powerful high voltage
source was begun. After a day of work, a new device
consisting of an automotive ignition coil and driving circuits therein
was made which delivered verymuch hotter sparks. Further
testing of the new device with the cannon proved this, and much more
consistent ignition was attained.

Click the picture to download a short video of one of the 'booms' produced.

I apologize for the crummy audio. I had the internal speaker connected to were
the mic goes and the mic connected to where the speaker goes.

Largely thanks to the new ignition source, a breakthrough has been made with the fuel
delivery system to turn those 'thumps' into 'booms' which can be felt hundreds of feet
distant. According to prior art which had been studied up to this point, the
frequency of the sound produced is directly related to the flame speed, also referred to
as deflagration/detonation velocity. Deflagration is a term generally understood to
mean a flame speed which is less than the speed of sound in air, while its more violent
cousin detonation pertains to flame speeds which are above the speed of sound in air.
This explains the difference between the 'boom' heard so far in my experiments and
the sharp pop which is produced by the agriculture device pictured above. This flame
speed is anticipated to be closely related to the fuel delivery system. Hopefully,
with further modifications, higher flame speeds will be achieved soon.

Update: 12/31/2006

Shawn Hughes writes:

>
> Hi!
>
> Found you through google. Did you ever work the kinks out in this? I was
> considering building one myself...
>
>
> Thanks in advance,
>
>
> -Shawn

Hello. Good to hear from you. I ran into some problems with the fuel delivery
system because it kept running too rich. It's funny you should ask me about
this now because I just solved this problem on Friday! See, the correct fuel
mixture is the difference between a thump, boom, or POW!!! When the weather
calms down and the wind quits blowing, I'll have to get out and do some more
testing on the full unit. I'm afraid if I test this indoors it'll blow out some
windows.